Method For Preparing Dissolving Pulp By Totally Chlorine-Free Bleaching Of Poplar Kraft Pulp

ABSTRACT

The application relates to a method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, including: preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as raw material; ZOQP bleaching of poplar kraft pulp to obtain dissolving pulp, wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching. In this way, washing and bleaching wastewater can be recycled in entire preparation process described above, achieving “zero” discharge. The dissolving pulp prepared by the method has no residual toxic substances, and exhibits that all indexes exceed the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and most of indexes meet those of excellent products; thus, it can replace imported dissolving pulp. The use of the method significantly solved the problems of large pollution in the production and shortage of raw materials for viscose fibers.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of Chinese Patent Application No. 201910796593.X, entitled “Method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp” filed with the Chinese national intellectual property administration on Aug. 27, 2019, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention belongs to the field of paper-making technology and new materials, and particularly relates to a method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp.

BACKGROUND ART

The information in background art is only intended to increase understanding of the overall background of the present invention, and is not necessarily regarded as an acknowledgement or any form of suggestion that constitutes prior art known to those skilled in the art.

Dissolving pulp is composed of high-purity cellulose, and is used to manufacture viscose fiber, acetate fiber, nitrocellulose, cellulose ether and other materials. Its important quality indexes include the content of α-cellulose, the content of hemicellulose, viscosity, molecular weight distribution, etc. In order to improve the quality of products, it is necessary to strictly control the contents of hemicellulose, lignin, ash and metal ions in dissolving pulp, and to improve the reactivity of the pulp and the molecular weight distribution of cellulose.

Hardwood has become the main fiber raw material for the production of dissolving pulp. The pre-hydrolysis kraft process has become the main method for the preparation of dissolving pulp. The popularity of oxygen-alkali delignification and ClO₂ bleaching has promoted the application of green bleaching processes such as ECF, light ECF and even TCF in the production of dissolving pulp.

However, the inventors have found that compared with grass-based pulp, wood-based pulp is less bleachable and more difficult to bleach. Domestic dissolving pulp enterprises still use traditional chlorine bleaching or elemental chlorine-free (ECF) bleaching to produce dissolving pulp. However, the traditional chlorine bleaching has the disadvantages of high consumption of bleaching water, difficulty in wastewater treatment, and the presence of carcinogenic substances such as dioxins. ECF bleaching has the problems of higher production costs, a large amount of acidic substances generated in the reaction resulting in lower pH at the end of bleaching, and strict equipment requirements, etc. Due to the limitation of factors such as cost and product quality, the current TCF bleaching effect in the preparation of dissolving pulp by using wood as raw materials is still difficult to meet the actual production requirements, the main indexes of dissolving pulp obtained by the same such as alkali resistance and D65 brightness are difficult to meet requirements of qualified products, and the preparation cost is relatively high.

SUMMARY

In order to overcome the above problems, the present invention provides an environmentally friendly method for preparing dissolving pulp by bleaching of pre-hydrolyzed poplar kraft pulp. Also, dissolving pulp prepared by the method exhibits that all indexes exceed the requirements of qualified products in the dissolving pulp industry standard (QB/T 4898-2015), and all indexes were superior to those of excellent products except for “methylene chloride extract”; thus, it can replace imported dissolving pulp.

In order to achieve the above object, the present invention adopts the following technical scheme.

A method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, comprising:

preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as a raw material;

performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp,

wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching.

After studying, it has been found that: the use of the totally chlorine-free bleaching, i.e. ZOQP bleaching to bleach poplar kraft pulp pretreated with hot water can obtain dissolving pulp which meet the requirements of qualified products with low drug consumption, achieve high yield of dissolving pulp, reduce production costs and bleaching wastewater discharge, and do not generate organic halide.

In some embodiments, the pre-hydrolysis kraft process includes procedures of hot water pretreatment and the kraft cooking. According to the invention, the pulp is prepared by the kraft process using the poplar pieces as raw materials, and then subjected to a new green and environmentally friendly totally chlorine-free (TCF) bleaching method, i.e. ZOQP, so that the wastewater generated by bleaching is completely free of harmful substances such as dioxin and AOX and has a greatly reduced COD content, achieving “zero” discharge with recycling of wastewater generated by washing and bleaching. The high-quality dissolving pulp produced by the method has no residual toxic substances, and exhibits that all its indexes exceed the requirements of qualified products in the dissolving pulp industry standard (QB/T 4898-2015), and most of the indexes meet those of excellent products. The use of the method significantly solved the problems of large pollution in the production of dissolving pulp and shortage of raw materials for viscose fibers.

In some embodiments, the hot water pretreatment comprises steps of: mixing poplar pieces with water in a liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min. The use of the hot water pretreatment can reduce the content of hemicellulose in the raw material, destroy the primary wall of the fiber, and facilitate the subsequent cooking of the poplar pieces.

In some embodiments, the kraft cooking comprises steps of: performing cooking for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180° C., the amount of alkali (calculated as Na₂O) of 20-25%, and the degree of vulcanization of 20-25%. The subsequently prepared dissolving pulp exhibits that all indexes exceeded the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and all indexes were superior to those of excellent products except for “methylene chloride extract”.

After studying, it has been found that O followed by Z is beneficial to the removal of residual lignin in the pulp after cooking, which is conducive to subsequent bleaching to obtain dissolving pulp with high whiteness, while Z followed by O can reduce the loss of cellulose in combination of ensuring high whiteness above the qualified product, and achieve higher yield of dissolving pulp. Therefore, in some embodiments, Z-stage includes steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%. The embodiments improve the yield and whiteness of dissolving pulp.

In some embodiments, O-stage includes steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO₃, and then performing the reaction at 90-110° C. for 30-50 min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%. Introducing of MgCO₃ could increase the whiteness of dissolving pulp, improve break length and tear factor, and decrease the content of polypentose.

In some embodiments, Q-stage includes steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA, mixing the pulp and DTPA uniformly, and performing a chelate treatment at 60-80° C. for 20-40 min. The use of the chelation treatment improves the whiteness of the dissolving pulp and the bleachability of P-stage.

In some embodiments, P-stage is performed as follows: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100° C. for 70-100 min; after the reaction is completed, the pulp is took out, and then is washed to neutrality and dried to obtain dissolving pulp. The prepared dissolving pulp has high quality and no toxic residues, and exhibits that all indexes exceed the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and most of the indexes meet those of excellent product, and thus it can replace imported dissolving pulp.

The invention also provides a dissolving pulp prepared by any of the methods described above.

The present invention also provides the application of the above-mentioned dissolving pulp for preparing viscose fiber, acetate fiber, nitrocellulose or cellulose ether.

The present invention has the following beneficial effects:

(1) The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp adopts hot water, oxygen, ozone, hydrogen peroxide, a small amount of alkali and traces of other auxiliaries. The wastewater generated by bleaching is completely free of harmful substances such as dioxin and AOX and has a greatly reduced COD content, achieving “zero” discharge with recycling of wastewater generated by washing and bleaching. The high-quality dissolving pulp prepared by the method has no residual toxic substances, and exhibits that all indexes exceed the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and most of the indexes meet those of excellent products, and thus it can replace imported dissolving pulp. The use of the method significantly solved the problems of large pollution in the production of dissolving pulp and shortage of raw materials for viscose fibers. The method is in line with the national industrial policy of resource conservation, energy conservation and emission reduction, and green development.

(2) The method of the present application is simple for operation and universal with low requirements on equipment and cost, and is easy for large-scale production.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanation of the present application. Unless defined otherwise, all technical and scientific terms used in the application have the same meaning as commonly understood by one of ordinary skill in the art to which the application belongs.

It should be noted that the terminology used herein is only for describing specific embodiments and is not intended to limit the exemplary embodiments according to the present application. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise, and it should also be understood that when the terms “comprising” and/or “including” are used in the specification, they indicate that there are features, steps, operations, devices, components, and/or combinations thereof.

As described in the background, the current TCF bleaching of wood dissolving pulp is not good in efficiency and has high cost. Therefore, the present invention provides an environmentally-friendly method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, in which poplar pieces are used as raw materials, comprising the following steps:

Cutting the naturally air-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm;

then, hot water pretreatment of the poplar pieces: mixing the poplar pieces with water according to the liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min;

subsequently, cooking the pre-treated poplar pieces by the kraft process for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180° C., the amount of alkali (based on Na₂O) of 20-25%, and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp; and

then, performing the totally chlorine-free ZOQP bleaching on the poplar kraft pulp (Z-ozone bleaching, O-oxygen delignification, Q-chelation treatment, P-hydrogen peroxide bleaching) to obtain dissolving pulp,

Z-stage includes steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%;

O-stage includes steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO₃, and then performing the reaction at 90-110° C. for 30-50 min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%;

Q-stage includes steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA , mixing the pulp and DTPA uniformly, and performing a chelate treatment at 60-80° C. for 20-40 min; and

P-stage includes steps of: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100° C. for 70-100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp. Various indexes of the dissolving pulp are measured.

All chemical reagents used in the present invention are chemically pure.

In the present application, the method for preparing dissolving pulp comprises steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp, and specifically comprises the following steps:

(1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

(2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water according to the liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min.

(3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 60-100 min at the liquid ratio of 1:4-1:7, the maintaining temperature of 150-180° C., the amount of alkali (based on Na₂O) of 20-25% and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain hot water pretreated poplar kraft pulp.

(4) Ozone bleaching (Z-stage): adjusting hot water pretreated poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.

(5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO₃, and then performing the reaction at 90-110° C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.

(6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.2-0.6% of chelating agent DTPA , mixing the pulp and DTPA uniformly, and then performing a chelate treatment at 60-80° C. for 20-40 min.

(7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100° C. for 70-100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

The specific steps comprise:

(1) Cutting the naturally air-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

(2) Mixing the poplar pieces obtained in step (1) with water according to the liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min.

(3) Cooking the poplar pieces pretreated in step (2) for 60-100 min at the liquid ratio of 1:4-1:7, the maintaining temperature of 150-180° C., the amount of alkali (based on Na₂O) of 20-25% and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain hot water pretreated poplar kraft pulp.

(4) Performing ozone bleaching (Z stage) on the pulp obtained in step (3): adjusting the pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of DTPA, mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.

(5) Performing oxygen delignification (O-stage) on the pulp obtained in step (4): placing the pulp in step (4) in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO₃, and then performing the reaction at 90-110° C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.

(6) Performing chelation treatment (Q-stage) on the pulp obtained in step (5): placing the pulp obtained in step (5) in a closed reactor, adding 0.3-0.6% of diethyltriaminepentaacetic acid (DTPA), mixing the pulp and DTPA uniformly, and then performing a chelate treatment in a water bath at 60-80° C. for 20-40 min while stirring at a speed of 60 r/min.

(7) Bleaching the pulp obtained in step (6) (P-stage) as follows: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100° C. for 70-100 min; after the reaction is completed, the pulp is taken out and washed to neutrality.

(8) Drying the pulp obtained in step (7) to obtain dissolving pulp, which is mainly used in the preparation of viscose fiber, acetate fiber, nitrocellulose or cellulose ether.

The technical solutions of the present application will be described below by specific examples. In the following examples, all percentages (%) refer to mass percentages.

EXAMPLE 1

Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

(1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

(2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:5, heating up to 165° C., and holding for 90 min.

(3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 90 min at the liquid ratio of 1:5, the temperature of 165° C., the amount of alkali (based on Na₂O) of 22% and the degree of vulcanization of 24%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

(4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 28% and placing it in a reactor, adding 2% of ozone and 0.5% of DTPA, adjusting the pH to 2.5 with acetic acid, and then performing the reaction at 25° C. for 20 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 10%.

(5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.6 Mpa, adding 2.5% of NaOH and 0.5% of MgCO₃, and then performing the reaction at the temperature of 100° C. for 40 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 10%.

(6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.5% of chelating agent DTPA, stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and performing a chelate treatment at 70° C. for 30 min.

(7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 2.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.8% of NaOH, 1.5% of sodium silicate and 0.2% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 90° C. for 90 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE 1 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance≥ % 93.0/90.0 93.1 Polypentose≤ % 4.0/4.5 3.5 Ash content≤ % 0.12/0.15 0.09 Iron content≤ mg/kg 15.0/20.0 14.8 D₆₅ brightness≥ % 82.0 82.7 Dichloromethane extract≤ % 0.30/0.40 0.37 Intrinsic viscosity mL/g 380~500/320~500 442 Dustiness 0.05 mm²~3.0 mm²/kg 60/80 42.4 mm²≤ >3.0 mm² none none

Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

EXAMPLE 2

Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

(1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

(2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:6, heating up to 170° C., and holding for 70 min.

(3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 100 min at the liquid ratio of 1:5, the temperature of 170° C., the amount of alkali (based on Na₂O) of 20% and the degree of vulcanization of 22%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

(4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 25% and placing it in a reactor, adding 1% of ozone and 0.3% of DTPA, adjusting the pH to 2.0 with acetic acid, and then performing the reaction at the temperature of 20° C. for 30 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 12%.

(5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.4 Mpa, adding 1.5% of NaOH and 0.3% of MgCO₃, and then performing the reaction at the temperature of 90° C. for 50 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 12%.

(6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.3% of chelating agent DTPA , stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and then performing a chelate treatment at 60° C. for 40 min.

(7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 1.5% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.6% of NaOH, 1.0% of sodium silicate and 0.1% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 95° C. for 100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE 2 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance≥ % 93.0/90.0 91.6 Polypentose≤ % 4.0/4.5 3.9 Ash content≤ % 0.12/0.15 0.13 Iron content≤ mg/kg 15.0/20.0 15.9 D₆₅ brightness≥ % 82.0 82.3 Dichloromethane extract≤ % 0.30/0.40 0.39 Intrinsic viscosity mL/g 380~500/320~500 481 Dustiness 0.05 mm²~3.0 60/80 60 47.7 mm²≤ >3.0 mm² none none

Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

EXAMPLE 3

Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

(1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces having a length of 3-6 cm and a thickness of 2-5 mm.

(2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:4, heating up to 175° C., and holding for 60 min.

(3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 80 min at the liquid ratio of 1:5, the temperature of 160° C., the amount of alkali (based on Na₂O) of 25% and the degree of vulcanization of 25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

(4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 30% and placing it in a reactor, adding 3% of ozone and 0.7% of DTPA, adjusting the pH to 3.0 with acetic acid, and then performing the reaction at the temperature of 25° C. for 10 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 15%.

(5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.8 Mpa, adding 3.5% of NaOH and 0.7% of MgSO₄, and then performing the reaction at the temperature of 110° C. for 30 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 15%.

(6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.6% of chelating agent DTPA , stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and performing a chelate treatment at 80° C. for 20 min.

(7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 1.0% of NaOH, 2.0% of sodium silicate and 0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 95° C. for 80 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE 3 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance≥ % 93.0/90.0 93.8 Polypentose≤ % 4.0/4.5 3.1 Ash content≤ % 0.12/0.15 0.09 Iron content≤ mg/kg 15.0/20.0 12.8 D₆₅ brightness≥ % 82.0 83.1 Dichloromethane extract≤ % 0.30/0.40 0.33 Intrinsic viscosity mL/g 380~500/320~500 401 Dustiness 0.05 mm²~3.0 60/80 60 39.9 mm²≤ >3.0 mm² none none

Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

Finally, it should be noted that the above examples are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the foregoing embodiments, it can still modify the technical solutions described in the foregoing embodiments, or equivalently replace some of them for those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. Although the specific embodiments of the present invention are described above, they do not limit the scope of protection of the present invention. Those skilled in the art should understand that based on the technical solution of the present invention, various modifications or deformations that can be made by those skilled in the art without creative effort are still within the protection scope of the present invention. 

What is claimed is:
 1. A method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, comprising: (1) preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as a raw material; and (2) performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp; wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching.
 2. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the pre-hydrolysis kraft process comprises procedures of hot water pretreatment and the kraft cooking.
 3. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the hot water pretreatment comprises steps of: mixing poplar pieces with water according to a liquid ratio of 1:4-1:7, heating up to 150-180° C., and holding for 60-100 min.
 4. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the kraft cooking comprises steps of: performing cooking for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180° C., the amount of alkali (calculated as Na₂O) of 20-25%, and the degree of vulcanization of 20-25%.
 5. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein Z-stage comprises steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35° C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.
 6. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein O-stage comprises steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO₃, and then and performing the reaction at 90-110° C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.
 7. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein Q-stage comprises steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA, mixing the pulp and DTPA uniformly, and then performing a chelate treatment at 60-80° C. for 20-40 min.
 8. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein P-stage comprises steps of: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction for 70-100 min at the temperature of 80-100° C.; after the reaction is completed, the pulp is took out, and then is washed to neutrality and dried to obtain dissolving pulp.
 9. A dissolving pulp prepared by the method according to claim
 1. 10. An application of dissolving pulp according to claim 9 in the preparation of viscose fiber, acetate fiber, nitrocellulose or cellulose ether. 